1. Field of the Invention
The present invention relates generally to a communication system, and in particular, to an apparatus and method for performing ranging in a communication system.
2. Description of the Related Art
In the 4th generation (4G) communication system which is the next generation communication system, active research is being conducted to provide services having various Qualities of Service (QoS) to users at a data rate of about 100 Mbps. Particularly, the current 4G communication system positively considers a communication system that uses an Orthogonal Frequency Division Multiplexing (OFDM) scheme and an Orthogonal Frequency Division Multiplex Access (OFDMA) scheme in order to support broadband high-speed transmission.
The OFDMA communication system needs a ranging procedure for matching an accurate time offset and adjusting power, between a transmitter and a receiver, for example, between a base station and a terminal. The ranging can be classified into three types: initial ranging, bandwidth request ranging, and periodic ranging.
Initial ranging is performed at the request of a terminal in order for the terminal to acquire synchronization with a base station. Initial ranging is performed to match an accurate time offset and adjust transmission power between the terminal and the base station.
Periodic ranging is periodically performed by a terminal to adjust a channel state with a base station after adjusting the time offset and transmission power with the base station through the initial ranging.
Bandwidth request ranging is performed by a terminal to request allocation of a bandwidth to perform actual communication with a base station after adjusting the time offset and transmission power with the base station through the initial ranging.
To perform the initial ranging procedure, the terminal transmits an arbitrarily selected ranging code to the base station. The initial ranging procedure will be described hereinbelow with reference to FIG. 1.
FIG. 1 shows a ranging signal received in a ranging symbol interval of a base station in a general communication system. Before a description of FIG. 1 is given, a description will be made of a TDD/OFDMA communication system that uses a Time Division Duplexing (TDD) scheme as a duplexing scheme, and an OFDMA scheme as a multiple access scheme, by way of example. In this communication system, the initial ranging is performed to acquire synchronization between the base station and the terminal in the state where uplink/downlink synchronization is not secured. However, when the ranging signal transmitted by the terminal of the communication system arrives in a ranging symbol interval of the base station, a synchronization error corresponding to a length of the Cyclic Prefix/Postfix (CP) inserted as a kind of a guard interval may occur in the ranging symbol interval.
When the synchronization error occurs between the base station and the terminal, the ranging signal cannot be restored. In addition, the ranging signal departing from the ranging symbol interval functions as an Inter-Symbol Interference (ISI) for the data signal that will arrive in the next symbol interval, deteriorating the system performance. Therefore, in the communication system, the terminal extends the ranging signal to a length of two symbols, for example, a length of two OFDM symbols (OFDMA symbols) in such a manner that phases thereof should be consecutive to each other, and then transmits the extended ranging signal so the ranging signal should always arrive before a start point of the ranging symbol interval.
Referring to FIG. 1, there are shown a first ranging symbol interval and a second ranging symbol interval of the base station, and each of the ranging symbol intervals corresponds to, for example, a size of one OFDM symbol. In addition, the ranging symbol interval includes a CP.
A ranging signal is shown that is transmitted by the terminal to the base station for ranging. Two OFDM symbols transmitted from the terminal through an uplink are allocated as the ranging signal, and the ranging signal is composed of two phase-consecutive OFDM symbols. The terminal transmits the ranging signal so the ranging signal should always arrive before the start point of the ranging symbol interval of the base station. That is, the terminal transmits the ranging signal so the ranging signal should always arrive before a start point the first ranging symbol interval. It does not matter because an interval given before the first ranging symbol interval starts is a guard interval.
The ranging signal received at the base station in this way is composed of two in-phase OFDM symbols obtained by repeating a ranging signal of a 1-OFDM symbol length. Therefore, the ranging signal arriving in the first ranging symbol interval is a ranging signal obtained by cyclic-shifting the ranging signal of a 1-OFDM symbol length. The base station estimates a time offset using the ranging signal received in the first ranging symbol interval.
A signal received in the second ranging symbol interval cannot be restored because it cannot have a 1-OFDM symbol length. In addition, since the signal received in the second ranging symbol interval serves as interference over the full band, it is not used.
As a result, when the foregoing ranging procedure is performed between the base station and the terminal, the ranging signal received in the second ranging symbol interval simply functions as a guard interval for ranging, thereby causing an unnecessary resource waste due to the ranging signal received in the second ranging symbol interval.